Each unit that you mentioned has it's strong points and draw backs.  Price is the advantage on the 500.  It is the cheapest unit by far and has the built in GPS.  Disadvantages are screen size and control capabilities.  The 500 will run the EZ-boom, but that is it as far as product control.  The 500 is also very limited in data logging.

The FMD and the InSight are very similiar hardware wise.  In fact they are essentially the same box.  But they differ greatly from the firmware side.  The FMD has one big advantage (if you want to move between vehicles).  All of the NAV2 calibrations can be done on the FMD.  You have to have a laptop or PDA with APToolbox to calibrate if you are using the InSight.  The AgLeader/Trimble relationship seems to have gotten sort of rocky lately.  Trimble will not let AgLeader do the calibrations and AgLeader will not let Trimble have yield monitoring capabilites.  The InSight has more control and monitoring capabilities than the FMD, but that is changing fast.  The FMD is gaining ground everyday with the current collaboration with Dickey-john.  The InSight will do much more than the FMD from a data logging standpoint.  The FMD logs to a polygon shape file.  That file is just about useless if you want to import into a true record keeping program.  It will give you a coverage map, but the map is a pretty picture and that is about it.  The InSight gives you much more data about product usage etc.

The Trimble NAV2  system is really not designed to be moved between vehicles.  While it is possible, I would not recommend it.  The tilt compensation is located in the NAV2 unit.  Every time that unit is moved, it will have to be recalibrated.  That calibration can be done on the FMD, but it takes time and room.  With the 500 or Insight, that calibration has to be done in ApToolbox which your dealer may or may not give you access to.  The displays and GPS can certainly be moved, but it is critical that the GPS be positioned in the same place every time it is returned to the vehicle.  That has a big effect on the calibration also.  You also have a nightmare of cables to move if you try to use this unit on more than one vehicle.  You can by separate harnesses, but they are expensive.

The FMD has new firmware this spring and most of the control and seed monitoring capabilities for the FMD are new this spring.  I have not operated the FMD this spring so I can't comment on how the  new stuff is doing.

In short, the choice between the different displays needs to be based on what you ultimately want to accomplish with the system.  If guidance is all you need the 500 may fit the bill.  If you are set on moving the unit between tractors and want control capabilities the FMD maybe the better fit with it's calibration capabilities.  If you want the ultimate in control and data logging is high on your list the InSight fits the bill.